Why is B+ low in my Super Reverb, but preamp plate voltage is normal? Can I run a GZ34 without making preamp plate voltage too high?

[itsGiusto]

I posted this deep in my NAD thread, where it got a little drowned out. I thought that maybe I'd get more advice if I promoted it to its own thread:


My new Super Reverb seems to be an AB763, however, it came with a 5U4 tube, not a GZ34 like it should if it were stock.

I took some voltages in the amp to see if I may want to do as @SoK66 suggested here and swap the 5U4 out for a GZ34, like would have come stock with this amp. Here are some of the voltages I found. Voltages were taken with amp fully on, all tubes and speakers plugged in, amp not in standby, and with volumes set to 0.

Voltage at 5U4 input: 359v AC
Voltage at 5U4 output (B+): 444v DC
Voltage at both power tube plates: 440v DC
Voltage at first preamp tube plate: 270v DC


Compare these against this schematic:

https://cdn.shopify.com/s/files/1/2325/2729/files/Super-Reverb-AB763-schematic.pdf?5926397210715640943

I find this confusing, because, the voltage input to the rectifier tube is correct (360v AC), and as @SoK66 suggested, the fact that there's a 5U4 there means that B+ is lower than it should be (444v instead of 465v). However, if this were the case, I would also expect that the preamp plate voltage would be lower as well, but instead we have a voltage of 270 at the plate, which matches the schematic perfectly.

Of note: I measured the values of the two dropping resistors in the dog house. They were 1k and 5.6k (even though the markings indicate 4.7k - it must have drifted over time). I didn't measure the choke, but I looked up the model (Mojo 777), and it seems to be the correct value (3H, 125C1A).

The dropping resistor confuses things even more: if the resistor has drifted to be increased in value, wouldn't that result in the plate voltage at the preamp tube being even lower? Why is the plate voltage at the preamp tube to spec, despite the dropping resistor difference and the rectifier tube difference?

Finally, given this, would it be a bad idea to put a GZ34 into this amp? Would the preamp tube voltage end up being too high?

Thanks, all!

 

[SoK66]

To understand how you are getting 440bdc at the plates (and I assume the same for B+) we need to know how the power tubes are biased. If they are biased "hot", like say 70% dissipation, that will reduce the B+. Conversely, if they are biased cold, like say at 50%, that will result in a higher B+. All things being equal, you can generally estimate the B+ difference between a 5U4 and a Gz34/5AR4 to be about 25 volts, so if you swapped them out you might see exactly 465v B+. All tubes aren't created equal, so one marked GZ34 and another same-same, they might individually render slightly different voltages.

The real difference between the two rectifiers is in my experience is the "feel." GZ34 is tighter and the signal "sags" less, a 5U4 has a bit of mush to it as it recovers. There are fans of each, no wromng answer. What works for you is the bullseye. Will also add, if this is any help to you, the original rectifier in my '68 Super Reverb was a Mullard GZ34. It lasted from new until 1991. I have a '66 Deluxe Reverb that is still on its original Mullard GZ34.

 

[itsGiusto]

To understand how you are getting 440bdc at the plates (and I assume the same for B+) we need to know how the power tubes are biased. If they are biased "hot", like say 70% dissipation, that will reduce the B+. Conversely, if they are biased cold, like say at 50%, that will result in a higher B+. All things being equal, you can generally estimate the B+ difference between a 5U4 and a Gz34/5AR4 to be about 25 volts, so if you swapped them out you might see exactly 465v B+. All tubes aren't created equal, so one marked GZ34 and another same-same, they might individually render slightly different voltages.

The real difference between the two rectifiers is in my experience is the "feel." GZ34 is tighter and the signal "sags" less, a 5U4 has a bit of mush to it as it recovers. There are fans of each, no wromng answer. What works for you is the bullseye. Will also add, if this is any help to you, the original rectifier in my '68 Super Reverb was a Mullard GZ34. It lasted from new until 1991. I have a '66 Deluxe Reverb that is still on its original Mullard GZ34.

Good question. They're actually biased pretty cold. It was 440v at the plate and 28ma current for a total of 41% dissipation (6L6GC).

I guess I'm mostly intereted in putting in a GZ34:
1. So I can hear how it sounded stock
2. Because I'm slightly worried about what you said of overtaxing the PT with the 5U4

 

[tubejockey]

My advice, don't worry about what the schematic says your voltage should be. Every component in the power supply and output stage has a tolerance that will affect your actual results. Don't worry about that.
What do you actually want to accomplish here? More headroom? Less sag? If so, you may want to go with a solid state rectifier, and re- bias accordingly. This may require a mod to the bias supply.
Don't worry about the difference in preamp voltage, that is negligible.

 

[Jasonpatrick]

Totally fine. Seen that many times. Least it’s on spec!

 

[SoK66]

Good question. They're actually biased pretty cold. It was 440v at the plate and 28ma current for a total of 41% dissipation (6L6GC).

I guess I'm mostly intereted in putting in a GZ34:
1. So I can hear how it sounded stock
2. Because I'm slightly worried about what you said of overtaxing the PT with the 5U4

If it's biased that cold and you're at 440v on the plates you can install a GZ34 and bias at maybe 60% and I'd bet be right in the zone. Give it a go!

I just looked at my notes on my '68 SR, these are the last bias settings, done when I swapped out the power tubes a couple eyars ago. Bear in mind I run the amp at 117vac using a buck transformer to reduce the line voltage. It has a old production Mullard GZ34 rectifier installed:

B+ / Plate = 442vdc
Bias = 42ma, 18.5w (62%)

I'd think if you installed a GZ34 in your amp at wall voltage the B+ would be in the 460 volt range with the bias at around 60% as I mentioned.

 

[itsGiusto]

If it's biased that cold and you're at 440v on the plates you can install a GZ34 and bias at maybe 60% and I'd bet be right in the zone. Give it a go!

I just looked at my notes on my '68 SR, these are the last bias settings, done when I swapped out the power tubes a couple eyars ago. Bear in mind I run the amp at 117vac using a buck transformer to reduce the line voltage. It has a old production Mullard GZ34 rectifier installed:

B+ / Plate = 442vdc
Bias = 42ma, 18.5w (62%)

I'd think if you installed a GZ34 in your amp at wall voltage the B= woul;d be in the 460 volt range with the bias at around 60% abs I mentioned.

Totally fine. Seen that many times. Least it’s on spec!

But if I installed a GZ34 and the preamp plates went up, how would I know if it's too high? What's safe for a 12AX7? If it went to 300v, would that be safe in this configuration?

 

[Jasonpatrick]

It won’t go up that much tbh.

 

[itsGiusto]

I just tried the swap from 5U4 to GZ34. It immediately brought the tube bias into a much better (and closer to Fender stock) range: plate voltage 454v, current 37.1 mv, for a total of 56% dissipation. B+ is now around 456v.

However, now the preamp plate voltage is 287v. That's not too high?

 

[Jasonpatrick]

I just tried the swap from 5U4 to GZ34. It immediately brought the tube bias into a much better (and closer to Fender stock) range: plate voltage 454v, current 37.1 mv, for a total of 56% dissipation. B+ is now around 456v.

However, now the preamp plate voltage is 287v. That's not too high?

270 is what’s on the “schematic” which you are still in the the fender ball park with 287V. Curious, what’s the last resistor down stream under the dog house? 4700? Check those both and see if they are still within tolerance. I think there is a 1k there too

 

[itsGiusto]

270 is what’s on the “schematic” which you are still in the the fender ball park with 287V. Curious, what’s the last resistor down stream under the dog house? 4700? Check those both and see if they are still within tolerance. I think there is a 1k there too

It's color coded as a 4.7k, but it reads as a 5.6k.

I tried out the amp after the switch, and the amp breaks up way less than it did before. Even if I run it at 10 (with the installed master volume at like 2, since it's late here), it barely gets any breakup. Previously, with the 5U4, it was getting significant breakup much lower on the dial, maybe at 5 or so.

I might prefer it with the 5U4, though obviously using the master volume so heavily skews my results. I'll have to try it with the MV turned all the way up at some point, to see what I think.

 

[TwoBear]

Isn’t there a higher current draw requirement on the 5U4 than the GZ34?

I would check it out but I’m on slow data plan-The new fast Wi-Fi comes back in tomorrow.

I’m thinking 1.5 or 1.9 for the GZ and could it be 3 A for the 5U4? I know the stock trannys are pretty stout, but if I’m not mistaken, that may be a little much to ask—1.1 - 1.9 Amps, and maybe cause it to run hot.

Hopefully I’m wrong, just thinking out loud.

 

[Jon Snell]

The anode voltages shown in the schematic are theoretical and will be widely different depending on the manufacture and dissipation of the pre amp valves. Four different triodes drawing exactly the same current ... not very likely.
In other words they will be fine. Slightly higher pre amp anode voltage means slightly less current drawn by the valve, a normal anomoly.

 

[Dacious]

The 5U4 drsws am extra amp of heater current but it's not a biggie. Your voltages are well within spec especially at today's higher walk voltages.

You can install a GZ34 as others have said and it will be less strain on the pt and a bit cleaner with more headroom.

You will get very little overdrive out of a standard AB763 preamp even on 10. Nearly all the distortion in these amps is the output transformer and speaker interaction at higher volumes

 

[itsGiusto]

The 5U4 drsws am extra amp of heater current but it's not a biggie. Your voltages are well within spec especially at today's higher walk voltages.

You can install a GZ34 as others have said and it will be less strain on the pt and a bit cleaner with more headroom.

You will get very little overdrive out of a standard AB763 preamp even on 10. Nearly all the distortion in these amps is the output transformer and speaker interaction at higher volumes

That's interesting you should say that. I get a nice crunchy overdrive out of my Ab763 Twin Reverb starting from 5 on the dial. But I will say that I've always noted that the B+ in that twin reverb is very low, and I don't know what causes it. Probably that's the reason there's overdrive in that amp.

 

[schmee]

I think you are good to go with the GZ34 if you want. Most of those old amps run way off from the schematic voltage wise. You can also pull V1 to lighten the load just a tad on the PT if you dont use the normal channel.

 

[itsGiusto]

That's interesting you should say that. I get a nice crunchy overdrive out of my Ab763 Twin Reverb starting from 5 on the dial. But I will say that I've always noted that the B+ in that twin reverb is very low, and I don't know what causes it. Probably that's the reason there's overdrive in that amp.

Also, actually, I just tried running the super reverb on 10 through my attenuator (which is also how I run the twin reverb to get it overdriven). That gave me nice crunch. So I guess you're right @Dacious, the distortion doesn't come from the preamp tubes in these amps, and it seems like it's possible it can come from the power tubes, so a master volume, even PPIMV, isn't going to help get crunch.